Certain phone covers or cases have been found to cause or contribute to microphone failures with the Cisco 8821 phone. Please discontinue using phone covers or cases to prevent damage to the microphone. External documentation can be found here - Cisco Wireless IP Phone 8821 Support for Third-Party Cases and Covers.
Note: This procedure can only be used in conjunction with the 8821 IP Phone earpiece. Speaker phone or the use of a headset is not supported. Step 4. Place a call to or from the 8821 phone, answer it, and mute the phone that is not being tested for microphone functionality.
Step 1. Swap out the battery in the phone that will not power on with another known working and charged battery and see if the phone will power on. Step 2. Test the battery from the phone that will not power on in another 8821 phone to see if the battery is functional.
If the 8821 phone charges only when the battery is changed with a known good battery, the battery is likely bad and will need to be replaced (part number CP-BATT-8821=). Please use the RMA Failure Code HW Fail - Power.